Build kernel. Make sure to set CONFIG_DRM_RADEON and add the relevant firmwares to CONFIG_EXTRA_FIRMWARE (e.g. "radeon/PALM_me.bin radeon/PALM_pfp.bin radeon/SUMO_rlc.bin radeon/SUMO_uvd.bin" for E-350).

Add "vdpau" to the USE flags and recompile packages that make use of it.

Reboot. Join forums / IRC / mailing lists for group hug.

UPDATE: Patchset v3 is out in the mailing list. Essentially the same as v2, with one change moved from 02 to 03.